1. Technical Field
The present invention relates to optical fiber sensors and, more specifically, to distributed fiber optical sensors where an external perturbation such as temperature, pressure, displacement, etc, introduces additional losses at different locations along the sensing fiber. Particularly, this invention relates to a distributed optical fiber sensor suitable to be used as an alarm-conditions detector in industrial, military and civilian systems.
2. Description of the Related Art
Distributed fiber optical sensors that can detect the loss-inducing perturbations are widely used in many different fields. For example, environmental sensors are used to detect pollutants and have become increasingly important as environmental standards. Chemical and petrochemical industry is another field where the use of sensors is growing, especially in the area of alarm-condition diagnostics. Further, sensors also used for temperature and strain measurements in various industrial applications.
The prior art includes many attempts to provide reliable detection of leaks of the contents of tanks, pipelines and the like. Detection of leaks in a simple, efficient and reliable fashion is highly desired at present because of increased public awareness of the sensitivity of the environment to chemical spills and the like. Moreover, increasing regulatory activity mandates reduction of industrial leakage of toxic chemicals and the like, and detection and cure of such leaks before their effects can become dangerous or catastrophic.
U.S. Pat. No. 5,378,889 to Lawrence entitled “Method and Apparatus for Detecting Hydrocarbon Fuels in a Vapor State with an Absorber-Expander Member” discloses a fiber optic sensor for the detection of hydrocarbon fuels. This type of prior art fiber optic sensor, however, is not optimal. The fiber optic sensor is inaccurate and limited in the distance over which it may be used.
U.S. Pat. No. 4,590,462 to Moorehead entitled “Oil leakage detection apparatus” employs microbending of an optical fiber in a detection unit to detect hydrocarbon fuels. A rotary actuator is mechanically coupled to an optical fiber to produce microbending of the fiber. The rotary actuator includes a spring mechanism having stored energy, which is released upon degradation of shear pins under the action of hydrocarbons. Thus, when the hydrocarbon analyte is present in sufficient quantity to degrade the shear pins, the spring is released and the optical fiber displaced to produce a microbend that can be sensed by optical time domain reflectometry. This approach, however, clearly is not reversible since it depends upon destruction of the shear pins upon contact with the hydrocarbon.
The prior art further shows a method for detecting the leak of an analyte by using a fiber optic sensor in conjunction with a substance that swells and mechanically creates a perturbation in, or particularly, a microbending in the optical fiber. The condition can then be readily detected by the attenuation in the signal transmitted by the fiber or by optical time domain reflectometer (OTDR).
For example, U.S. Pat. No. 5,138,153 to Gergely et al. entitled “Distributed Fiber-Optic Sensor with Substance Selective Permeable Coating” discloses a fiber optic sensor based upon passing effects in which the cladding has an index of refraction less than the core, and the cladding is sensitized to the analyte. When the analyte contacts the cladding, it increases the index of refraction of the cladding above the core to thereby couple the light transmitted in the core to the evanescent wave. The Gergely et al. patent employs its sensor system in a hydrocarbon tank farm, but the cladding is selected to undergo an increase in the index of refraction. Optical time domain reflectometry is used to locate leaks, and both continuous and pulsed light can be employed to sense liquids and vapors having analytes, which will react with the cladding. The Gergely reference detects the leak in the pipeline, but not the location of the leak.
U.S. Pat. No. 5,015,843 to Seitz et al. entitled “Fiber Optic Chemical Sensors Based on Polymer Swelling” is directed to a fiber optic system in which polymer swelling is used to mechanically or physically displace a reflective surface coupled to the fiber optic core and thereby influence light transmission back to the detector. The system requires a relatively high concentration of analyte to be effective, and in order to enhance sensitivity and minimize this disadvantage, the system preferably is miniaturized.
In some environments, it is necessary or desirable to monitor the location and magnitude of selected loads acting upon a physical structure.
U.S. Pat. No. 4,421,979 to Asawa et al., entitled “Microbending of Optical Fibers for Remote Force Measurement” discloses a system for remote measurement of structural forces, including a plurality of microbend transducers mounted along the length of the structure for microbending an optical fiber in response to structural forces.
All prior art distributed sensors use for the localization of number of consecutive perturbations time or frequency domain analysis. In general, prior art detectors have been unduly complicated, and thus, both expensive and prone to erroneous signals.
U.S. Pat. No. 6,542,228 to Hartog, entitled “Optical time domain reflectometry method and apparatus” discloses a system for the localization of number of the consecutive perturbations using optical time-domain reflectometry (OTDR)
U.S. Pat. No. 4,840,481 to Spillman, Jr, entitled “Polarimetric optical frequency domain distributed strain sensor and method” and U.S. Pat. No. 4,859,843 to Baney et al., entitled “Method and apparatus for optical signal analysis using a gated modulation source and an optical delay circuit to achieve a self-homodyne receiver” disclose a system which utilizes optical frequency domain reflectometry (OFDR) for the localization of the number of consecutive perturbations. The authors of these inventions do not teach how to localize the number of the perturbations using unmodulated light flux.
United States Patent Application 20030052256 to Spirin et al, entitled “Fiber optic sensor with transmission/reflection analyzer” discloses a fiber optical distributed sensor for the localization of the single perturbation using non-modulated light flux. However, the invention does not describe how to localize a number of the perturbations that can affect testing fiber consequently or simultaneously.
Accordingly, there is a need in the art for a system and device for detecting and localizing number of consecutive loss-inducing perturbations, which system and device are comparatively easy to manufacture, inexpensible, and reliable.
Accordingly, it is an object of the present invention to provide an alarm sensor and method for its use which is adaptable to a wide range of applications, is suitable for sensing a number of consecutive loss-inducing perturbations.
The present invention overcomes the problems encountered in the prior art by providing a practical and effective optical fiber sensor for detecting the magnitude and location of any number of consecutive perturbations acting along the length of a structure.